ON LIGHT AND COLOURS. 155 



example of the rest, at 4 feet from the third edge the chart 

 was fixed and the third edge kept constantly at that distance 

 from it. Then the double-edge instrument was placed suc- 

 cessively at 14i, at 9 and at 4^- eighths of an inch from the 

 third edge. The breadths were respectively 2, 3-| and 4i 

 twentieths of an inch, In some experiments these measures 

 approached more nearly the hyperbolic values of y, but I give 

 the experiment now only for the important and indeed 

 decisive evidence which it affords, that these fringes are 

 caused by disposition, and are wholly different from those 

 formed without previous flexion. 



Exp. 3. If the greater breadth of these fringes is owing to 

 dispersion, then they should be formed more in the rays of 

 the prismatic spectrum than in white light, or even in light 

 bent by flexion. Yet we find it more difficult to trace fringes 

 across the prismatic spectrum than in white light, and more 

 difficult across the spectrum when there is divergence, than 

 when formed parallel to its sides when there is no divergence. 

 There are fringes formed, but of the narrow kind, which are 

 described in Prop. I. 



Exp. 4. I have tried the effect on the fringes in question of 

 the curvilinear edge described in the first article of these 

 observations, and the effect of which is represented in fig. 18. 

 It is certain that at a distance from the doiible-edge instru- 

 ment the third edge seems only to form fringes rectilinear, or 

 of its own form. But when placed very near, as half an inch 

 from the instrument, plainly there is a curvilinear form given 

 to the fringes in question ; and this is most easily perceived, 

 when, by moving the third edge towards the side of the 

 pencil, you form the smaller fringes so as to be drawn across 

 or along the greater ones made by the two first edges. 



I think, without pursuing this subject further, it must be 

 admitted that these fringes in light, which is bent and dis- 

 posed, lend an important confirmation to the doctrine of 

 disposition. It is clear that the rays are aifected only on two 

 of their four sides, or a b and c d, if these are parallel to the 

 bending body's edge, and not at all on the sides cb and da; 



